add shipping costs with live exchange rates and country detection

Shipping rates fetched from Printify during product sync, converted to
GBP at sync time using frankfurter.app ECB exchange rates with 5%
buffer. Cached in shipping_rates table per blueprint/provider/country.

Cart page shows shipping estimate with country selector (detected from
Accept-Language header, persisted in cookie). Stripe Checkout includes
shipping_options for UK domestic and international delivery. Order
shipping_cost extracted from Stripe on payment.

ScheduledSyncWorker runs every 6 hours via Oban cron to keep rates
and exchange rates fresh. REST_OF_THE_WORLD fallback covers unlisted
countries. 780 tests, 0 failures.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

test/simpleshop_theme/sync/scheduled_sync_worker_test.exs

@@ -0,0 +1,40 @@
+defmodule SimpleshopTheme.Sync.ScheduledSyncWorkerTest do
+  use SimpleshopTheme.DataCase, async: false
+  use Oban.Testing, repo: SimpleshopTheme.Repo
+
+  alias SimpleshopTheme.Sync.ScheduledSyncWorker
+  alias SimpleshopTheme.Sync.ProductSyncWorker
+
+  import SimpleshopTheme.ProductsFixtures
+
+  describe "perform/1" do
+    test "enqueues sync for enabled connections" do
+      conn = provider_connection_fixture(%{enabled: true})
+
+      Oban.Testing.with_testing_mode(:manual, fn ->
+        assert :ok = perform_job(ScheduledSyncWorker, %{})
+
+        assert_enqueued(
+          worker: ProductSyncWorker,
+          args: %{provider_connection_id: conn.id}
+        )
+      end)
+    end
+
+    test "skips disabled connections" do
+      _conn = provider_connection_fixture(%{enabled: false})
+
+      Oban.Testing.with_testing_mode(:manual, fn ->
+        assert :ok = perform_job(ScheduledSyncWorker, %{})
+        refute_enqueued(worker: ProductSyncWorker)
+      end)
+    end
+
+    test "handles no connections gracefully" do
+      Oban.Testing.with_testing_mode(:manual, fn ->
+        assert :ok = perform_job(ScheduledSyncWorker, %{})
+        refute_enqueued(worker: ProductSyncWorker)
+      end)
+    end
+  end
+end